Optical recording medium and optical recording/reproducing method

ABSTRACT

The present invention provides an optical recording medium that includes a recording layer composed mainly of an organic compound and can utilize blue-violet semiconductor laser light (390 to 420 nm in wavelength) as recording/reproducing laser light. The present invention also provides an optical recording/reproducing method using the optical recording medium. The optical recording medium  1  comprises at least a supporting substrate  2 ; a recording layer  3  on the supporting substrate  2 , the recording layer  3  containing an organic compound as a major component; and a light-transmitting layer  5  on the recording layer  3 , the light-transmitting layer  5  being capable of transmitting laser light with a wavelength of 390 to 420 nm for recording and reproducing information. The organic compound in the recording layer  3  includes a trimethine cyanine dye that has the minimum value n min  of its refractive index n (real part of the complex refractive index) within the range of 370 to 425 nm and has a refractive index n of 1.2 or lower with respect to the wavelength of the recording/reproducing laser light. The organic compound, when absorbing the laser light, melts or degrades to bring about a change in the refractive index, thereby effecting recording of the information.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical recording medium thatincludes a recording layer composed mainly of an organic compound, andto an optical recording/reproducing method using the optical recordingmedium.

2. Disclosure of the Related Art

Rewritable media for optical recording of information, such as CD-Rs(compact disk-recordable) and DVD-Rs (digital versatiledisk-recordable), have become widely used. Such recording media includea recording layer that makes use of an organic dye. CD-R, a recordingmedium that permits the use of near infrared laser light inrecording/reproduction of information, offers advantageouscharacteristics, such as low prices and ability to keep recordedinformation from being altered, that have made this recording mediumwidely accepted in the marketplace. In response to an increasing demandfor high-density recording medium that offers long recording time,DVD-Rs have been developed and become increasingly popular. DVD-Rsenable long-time recording by permitting the use of infrared laser lightin recording/reproducing of information: DVD-Rs can utilizerecording/reproducing laser light with a wavelength of 650 nm, ascompared to 780 nm used in CDs, and optical systems employed in DVD-Rsachieve a numerical aperture (referred to as ‘NA,’ hereinafter) of 0.6,as compared to 0.45 for CDs. In this manner, DVD-Rs have achieved alarge recording capacity (4.7 GB/per side), which is 6 to 8 times largerthan that of CDs and allows about 2-hour recording time when typical TVsignals are recorded.

To meet a demand for recording media with even higher recording density,efforts have been made to further decrease the wavelength of therecording/reproducing laser light and increase the numerical aperture ofthe optical system employed. For example, an advanced optical recordingmedium is proposed for use in a system in which blue-violetsemiconductor laser light (390 to 420 nm in wavelength) is used as therecording/reproducing laser light in conjunction with a lens system withan NA of 0.76 or higher. The use of the short wavelength laser light isexpected to bring about a significant increase in the recording densityof optical recording media.

Various organic dye materials have been proposed for use in therecording layers of CD-Rs and DVD-Rs. Some of these materials that havealready been put to practical use are such that the long wavelengthregion of their absorption spectrum corresponds to the wavelength of therecording/reproducing laser light and the requirements for both the highrefractive index (n>2.0) and the proper extinction coefficient(0.01<k<0.10) are met in the long wavelength region. When the recordinglaser light is irradiated onto the recording layer, the organic dyematerial absorbs the light and as a result, melts or degrades, which inturn causes the refractive index of the recording layer to decrease froma relatively high value to a relatively low value. As a result,recording pits are formed to record information. To subsequentlyreproduce the recorded information, the reproducing laser is shone ontothe recording layer and the information is read by taking advantage ofthe difference in the reflective index with respect to the laser lightbetween the recording pit and the surrounding non-recording area.

CD-Rs and DVD-Rs are both required to have a high reflective index inorder to ensure compatibility with CD-ROMs (read-only memory) andDVD-ROMs, both of which have a high reflective index. However, theorganic dye material that has a refractive index of 2<n<3 and anextinction coefficient of 0.01<k<0.10 cannot provide a desired highreflective index by itself. For this reason, CD-Rs and DVD-Rs include ahighly reflective metal reflection layer on one side of the recordinglayer opposite from the side exposed to the laser light. In short, forthe purpose of ensuring a high reflective index and a high modulationand thus ensuring the compatibility with ROMs, CD-Rs and DVD-Rs bothemploy an organic dye material that, when illuminated with light in thewavelength range of the recording/reproducing laser, melts or degradesto change its refractive index from a relatively high value to arelatively low value.

The advanced optical recording media, in which blue-violet semiconductorlaser light (390 to 420 nm in wavelength) is use as therecording/reproducing laser light, generally have a low reflectiveindex, since there is difficulty in principle in imparting a highreflectance comparable to that of ROMs to the rewritable media (RW),which use a phase-change material in their recording layers. Thus, if arewritable optical recording medium is developed that, unlike CD-Rs andDVD-Rs, does not require a high reflective index, it will becomepossible to use, in the recording layer of the recording medium, anorganic dye material that, when irradiated with laser light, melts ordegrades to cause the refractive index to change from a relatively lowvalue to a relatively high value. This possibility is suggested inJapanese Patent Laid-Open Publication No. 2001-273672.

SUMMARY OF THE INVENTION

However, no organic dye materials have been known thus far that melts ordegrades to cause the refractive index to change from a relatively lowvalue to a relatively high value by the wavelength range of therecording/reproducing laser of 390 to 420 nm. Also, unlike the case withCD-Rs and DVD-Rs, it is generally considered difficult to adapt thelonger wavelength region of the absorbance spectrum to the range of 390to 420 nm. While some UV-absorbing agents are known to have the longerwavelength region of their absorbance spectrum within the range of 390to 420 nm, the relatively short conjugate system of, and thus therelatively small molecular size of, the UV-absorbing agents make themless soluble in an organic solvent. Not only does this make UV-absorbingagents unsuitable for use in spin-coating, but it also makes themsusceptible to crystallization when the agents are formed into a thinfilm.

Accordingly, it is an objective of the present invention to provide anoptical recording medium that includes a recording layer composed mainlyof an organic compound and can utilize blue-violet semiconductor laserlight (390 to 420 nm in wavelength) as the recording/reproducing laserlight. It is also an objective of the present invention to provide anoptical recording/reproducing method using the optical recording medium.

Thus, one aspect of the present invention provides an optical recordingmedium comprising at least a supporting substrate; a recording layer onthe supporting substrate, the recording layer containing an organiccompound as a major component; and a light-transmitting layer on therecording layer, the light-transmitting layer being capable oftransmitting laser light with a wavelength of 390 to 420 nm forrecording and reproducing information, wherein the organic compound inthe recording layer includes a trimethine cyanine dye that has theminimum value n_(min) of its refractive index n (real part of thecomplex refractive index) within the range of 370 to 425 nm and has arefractive index n of 1.2 or lower with respect to the wavelength of therecording/reproducing laser light, and the organic compound, whenabsorbing the laser light, melts or degrades to bring about a change inthe refractive index, thereby effecting recording of the information.

The present invention is the above-described optical recording medium,wherein, at the wavelength of the reproducing laser light, the meltingor the degradation of the organic compound causes an increase in therefractive index n of the organic compound.

The present invention is the above-described optical recording medium,wherein the organic compound has an extinction coefficient k (imaginarypart of the complex refractive index) of 0.15 or above, with respect toboth the wavelength of the recording laser light and the wavelength ofthe reproducing laser light.

The present invention is the above-described optical recording medium,wherein the trimethine cyanine dye contains a trimethine chain with twonitrogen-containing heterocyclic rings positioned on ends of thetrimethine chain, one of the two nitrogen-containing heterocyclic ringsbeing selected from the group consisting of benzoxazole, thiazoline, andthiazole, and the other of the two heterocyclic rings being selectedfrom the group consisting of benzoxazole, benzimidazole, indolenine,thiazoline, and thiazole.

The present invention is the above-described optical recording medium,wherein the trimethine cyanine dye contains a trimethine chain with twonitrogen-containing heterocyclic rings positioned on ends of thetrimethine chain, the two nitrogen-containing heterocyclic rings beingidentical to one another.

The present invention is the above-described optical recording medium,wherein the recording layer contains, in addition to the organiccompound, a quencher.

The present invention is the above-described optical recording medium,in which lands and grooves are formed on the supporting substrate withthe grooves being 60 to 150 nm in depth. In another embodiment, thepresent invention is the above-described optical recording medium, inwhich only the land area serves as the recording area.

The present invention is the above-described optical recording medium,which comprises a dielectric layer on the recording layer and thelight-transmitting layer on the dielectric layer. In another embodiment,the present invention is the above-described optical recording medium,in which the dielectric layer has a refractive index n₄ (real part ofthe complex refractive index) of 2 or higher and an extinctioncoefficient k₄ (imaginary part of the complex refractive index) of 0.2or lower with respect to the wavelength of the recording/reproducinglaser light.

The present invention is the above-described optical recording medium,in which the light-transmitting layer has a thickness of 1 μm to 150 μmin the signal recording/reproducing region.

Another aspect of the present invention is an opticalrecording/reproducing method, comprising the steps of:

providing an optical recording medium comprising at least a supportingsubstrate; a recording layer on the supporting substrate, the recordinglayer containing an organic compound as a major component; and alight-transmitting layer on the recording layer, the light-transmittinglayer being capable of transmitting laser light with a wavelength of 390to 420 nm for recording and reproducing information, wherein the organiccompound in the recording layer includes a trimethine cyanine dye thathas the minimum value n_(min) of its refractive index n (real part ofthe complex refractive index) within the range of 370 to 425 nm and hasa refractive index n of 1.2 or lower with respect to the wavelength ofthe recording/reproducing laser light, and the organic compound, whenabsorbing the laser light, melts or degrades to bring about a change inthe refractive index;

irradiating a recording laser light of 390 to 420 nm onto the opticalrecording medium from the light-transmitting layer side thereof toeffect recording of the information, whereupon the refractive index n ofthe organic compound with respect to the wavelength of reproducing laserlight of 390 to 420 nm is raised in the area irradiated with therecording laser light; and

subsequent to the recording step, irradiating the reproducing laserlight of 390 to 420 nm onto the optical recording medium from thelight-transmitting layer side thereof to effect reproducing of theinformation.

According to the present invention, there is provided an opticalrecording medium including an organic recording layer that allowsrecording/reproducing of information with a high sensitivity and a highdegree of modulation by using blue-violet semiconductor laser light (390to 420 nm in wavelength) as the recording/reproducing laser light.

BRIEF DESCRIPTION OF THE DRAWING

FIGURE is a schematic cross-section view showing main elements of oneexemplary construction of an optical disk of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

An optical recording medium (referred to simply as ‘optical disk,’hereinafter) of the present invention will now be described withreference to the accompanying drawing.

Referring to the FIGURE, one exemplary construction of the optical diskof the present invention is shown in a schematic cross-section. In theFIGURE, the optical disk (1) includes a recording layer (3), adielectric layer (4) and a light-transmitting layer (5) in this order ona surface of a supporting substrate (2) where information pits,pregrooves and other fine features are formed. The laser light upon theoptical disk (1) passes through the light-transmitting layer (5) torecord or reproduce the information.

The supporting substrate (2) is 0.3 to 1.6 mm thick and preferably, 0.5to 1.3 mm thick, and includes information pits, pregrooves, lands, andother fine features formed on the same surface where the recording layer(3) is formed. In the present invention, the grooves (G) functioned asguide slots are positioned closer to the side where laser light isirradiated (light-transmitting layer (5) side), and the grooves (G) areformed between the lands (L). The grooves (G) are generally formed asspirals.

The depth (Gd) of the groove (G) is defined as the difference in heightbetween the highest point of the land (L) and the lowest point of thegroove (G) and is preferably in the range of 40 to 150 nm, and morepreferably in the range of 60 to 120 nm. By setting the depth (Gd) ofthe groove (G) within this range, sufficient tracking control isachieved and crosstalk can be prevented. If formed with a depth (Gd) ofless than 40 nm, the grooves (G) are inclined to result in a decrease intracking error signals, which are required for following the tracks, anincrease in crosstalks, and a decrease in wobble signals and otherpreformatted signals. On the other hand, the grooves (G) with a depth(Gd) greater than 150 nm make it difficult to accurately form the lands(L) and the grooves (G) and may result in reduced reflection signals andreduced sensitivity.

The width (Gw) of the groove (G) is defined as a width of the groovemeasured at half the groove depth (Gd) and is preferably in the range of110 to 210 nm, and more preferably in the range of 130 to 190 nm. Thepitch (Gp) of the grooves (G) is defined as a distance between adjacentgrooves and may be defined as a distance between midpoints of theadjacent grooves taken along the width (Gw). The groove pitch (Gp) isfor example in the range of 290 to 350 nm, and preferably in the rangeof 310 to 330 nm. Such construction is effective in preventingcrosstalks.

While the land-and-groove recording method or the groove recordingmethod may be employed in recording information on the optical recordingmedium of the present invention, the land recording method, by whichonly the lands are used as recording areas, is preferred. When thesupporting substrate (2) has the above-described construction and therecording layer (3) composed mainly of the organic compound is formedover the supporting substrate (2) using spin-coat technique, therecording layer (3) tends to be formed with a larger thickness in theland areas than in the groove areas with more organic compound formed inthe land areas. For this reason, it is preferred to use the lands asonly recording areas.

Materials for the supporting substrate (2) should not necessarily beoptically transparent and may include various plastic materials such aspolycarbonate resin and polymethyl methacrylate (PMMA) and other acrylresins, and polyolefin resin. The use of such flexible materials isparticularly effective in the present invention since the substrate canbe kept from warping. This, however, does not exclude the use of glass,ceramics, and metals. The features are mostly formed by injectionmolding when a plastic material is used and by photopolymer technique(2P technique) when other materials are used.

The recording layer (3), which contains the organic compound as a majorcomponent, is formed over the supporting substrate (2). The organiccompound has the minimum value n_(min) of its refractive index n (realpart of the complex refractive index) within the range of 370 to 425 nmand includes a trimethine cyanine dye that has a refractive index n of1.2 or lower with respect to the wavelength of the recording/reproducinglaser light. The trimethine cyanine dye, when absorbing the recordinglaser light with a wavelength of 390 to 420 nm, melts or degrades tocause a change in the refractive index. By saying “the recording layercontains the organic component as a major component,” it is meant that,aside from essential components, the recording layer is composed of theorganic compound. The recording layer may, however, contain organiccompounds other than the organic compound that has the characteristicsmentioned above (for example, a quencher) and may also contain certaininorganic compounds in an amount of 10% or less by weight for thepurpose of improving properties of the layer.

The refractive index n (real part of the complex refractive index) forthe recording laser light of 390 to 420 nm is selected to be 1.2 orbelow, so that, upon recording, the organic compound, absorbing therecording laser light of 390 to 420 nm, melts or degrades to cause therefractive index to change from a relatively low value to a relativelyhigh value (for example, 1.45 to 1.65). In this manner, recording pitsare formed to record information. Upon reproduction of information, theinformation is read by taking advantage of the difference in thereflective index for the reproducing laser light of 390 to 420 nmbetween the recording pit and the surrounding non-recording region. Onthe basis of this principle, recording with the recording laser light of390 to 420 nm and reproduction with the reproducing laser light of 390to 420 nm can be effected. To cause a more significant change in therefractive index, the minimum value n_(min) of the refractive index nwithin the range of 370 to 425 nm is preferably selected to be 1.1 orlower, and more preferably 1.0 or lower. Although there is no particularlower limit for the minimum value n_(min), it is typically selected tobe approximately 0.7.

The extinction coefficient k (imaginary part of the complex refractiveindex) of the subject organic compound is preferably 0.15 or larger, andmore preferably 0.3 or larger, for the wavelength range of both therecording laser light and the reproducing laser light. The extinctioncoefficient k for the wavelength of the recording laser light of 0.15 orlarger allows the recording laser light to be properly absorbed by thearea on which the recording pits are formed. This causes the localtemperature to rise and facilitates melting or degradation of theorganic compound, which in turn causes the refractive index to change.In contrast, the extinction coefficient k for the wavelength of therecording laser light of less than 0.15 will result in a reducedabsorption of the recording laser light, making it difficult to effectrecording with normal recording power. On the other hand, the extinctioncoefficient k for the wavelength of the reproducing laser light of 0.15or larger provides a desired reflective index in the non-recordingareas. This helps to detect the difference in the reflective indexbetween the recording pits and the non-recording area. However, theextinction coefficient k for the wavelength of the reproducing laserlight is preferably kept at 0.95 or smaller since too large anextinction coefficient k may cause a reduced reflective index. For thesereasons, the extinction coefficient k of the subject organic compound ispreferably in the range of 0.3 to 0.95, and more preferably in the rangeof 0.4 to 0.8, with respect to the wavelength of both of the recordinglaser light and the reproducing laser light.

In the present invention, the refractive index n (real part of thecomplex refractive index) and the extinction coefficient k (imaginarypart of the complex refractory index) of the organic compound aremeasured based on the absorption-spectrum of the organic compound in theform of thin film. In general, the absorption spectrum of a thin film isobtained in the following manner: an organic compound for which todetermine the absorption spectrum is dissolved in a suitable organicsolvent. Using spin-coat technique, the resulting solution is appliedonto a groove- or pit-free polycarbonate plate to a film thickness ofabout 40 to 100 nm. Care must be taken to keep the organic compound fromhaving a particular orientation. Should the organic compound undergosignificant crystallization or association during evaporation of thesolvent in the spin-coat process, a different solvent may be used. Usinga spectrophotometer, the resulting polycarbonate substrate with a thinfilm of the organic compound was measured for the transmission andabsorbance spectra.

In the present invention, a trimethine cyanine dye that meets therequirements for the above-described conditions is used to serve as theabove-described organic compound. Provided that the above-describedconditions are met, the trimethine cyanine dye is selected from thoseexpressed by the following general formula (I):

wherein Q and Q′ may or may not be identical to one another and eachindependently represent an atom group forming a nitrogen-containingheterocyclic rings, which may be a condensed ring and may be substitutedor unsubstituted; Y represents a hydrogen atom, a halogen atom, a loweralkyl group such as methyl, or a phenyl group; R₁ and R₁′ may or may notbe identical to one another and each independently represent asubstituted or unsubstituted alkyl group having 1 to 6, preferably 1 to4, carbon atoms (i.e., methyl, ethyl, propyl, and butyl); X⁻ representsan anion, which may be a halogen ion such as Cl⁻, Br⁻, and I⁻, ClO₄ ⁻,BF₄ ⁻, PF₆ ⁻, SbF₆ ⁻, or SCN⁻; and m is 0 or 1.

In the general formula (I), the nitrogen-containing heterocyclic ring oneither end of the trimethine chain may be a benzoxazole (A),benzimidazole (B), indolenine (C), thiazoline (D), or thiazole (E), asrepresented by the following general formulae (in each formula, N isconveniently shown in the charged state):

In the general formula (A) representing a benzoxazole, R₁ is asubstituted (with, for example, an alkoxy group) or unsubstituted alkylgroup having 1 to 4 carbon atoms (i.e., methyl, ethyl, propyl, andbutyl, with propyl and butyl particularly preferred). R₂, R₃, R₄ and R₅may or may not be identical to one another and each independentlyrepresent a hydrogen atom, alkyl group, nitro group, alkoxy group, or ahalogen atom such as Cl.

In the general formula (B) representing a benzimidazole, R₁ is asubstituted or unsubstituted alkyl group having 1 to 4 carbon atoms(i.e., methyl, ethyl, propyl, and butyl, with propyl and butylparticularly preferred). R₆ represents a methyl or ethyl group. R₂, R₃,R₄ and R₅ may or may not be identical to one another and eachindependently represent a hydrogen atom, alkyl group, nitro group,alkoxy group, or a halogen atom such as Cl.

In the general formula (C) representing an indolenine, R₁ is asubstituted (with, for example, an alkoxy group) or unsubstituted alkylgroup preferably having 1 to 4 carbon atoms (i.e., methyl, ethyl,propyl, and butyl). R₇ and R₈ may or may not be identical to one anotherand each independently represent a methyl or ethyl group. R₂, R₃, R₄ andR₅ may or may not be identical to one another and each independentlyrepresent a hydrogen atom, alkyl group, nitro group, alkoxy group, or ahalogen atom such as Cl.

In the general formula (D) representing a thiazoline, R₁ is asubstituted (with, for example, an alkoxy group) or unsubstituted alkylgroup having 1 to 4 carbon atoms (i.e., methyl, ethyl, propyl, andbutyl, with propyl and butyl particularly preferred). R₉, R₁₀, R₁₁ andR₁₂ may or may not be identical to one another and each independentlyrepresent a hydrogen atom, alkyl group, or a halogen atom such as Cl.

In the general formula (E) representing a thiazole, R₁ is a substituted(with, for example, an alkoxy group) or unsubstituted alkyl group having1 to 4 carbon atoms (i.e., methyl, ethyl, propyl, and butyl, with propyland butyl particularly preferred). R₁₃ and R₁₄ may or may not beidentical to one another and each independently represent a hydrogenatom, alkyl group, or a halogen atom such as Cl.

In the general formula (I), one of the two nitrogen-containingheterocyclic rings positioned on ends of the trimethine chain isselected from the group consisting of the benzoxazole (A), thethiazoline (D) and the thiazole (E), and the other of the twoheterocyclic rings is selected from the group consisting of thebenzoxazole (A), the benzimidazole (B), the indolenine (C), thethiazoline (D), and the thiazole (E). The trimethine cyanine dye isparticularly preferred when the two nitrogen-containing heterocyclicrings on ends of the trimethine chain are identical to one another togive the dye a symmetrical structure. Specifically, trimethine cyaninedyes of symmetrical structure with both of the two nitrogen-containingheterocyclic rings being one selected from the group consisting of thebenzoxazole (A), the thiazoline (D) and the thiazole (E) are morepreferred. The trimethine cyanine dyes with symmetrical structure tendto have a smaller refractive index n (real part of the complexrefractive index) in the range of 370 to 425 nm as compared to thosewith asymmetrical structure. This is preferable since significantmodulation can be achieved before and after recording. Two or moredifferent trimethine cyanine dyes may be used together for the purposesof adjusting the refractive index n and the extinction coefficient k andenhancing the solubility of the dye.

More specifically, the following trimethine cyanine dyes may be used:

These trimethine cyanine dyes may be used either individually or incombination of two or more dyes or may be used in conjunction with asinglet oxygen quencher (which will be described later) so as to obtaindesired values for the refractive index n (real part of the complexrefractive index) and the extinction coefficient k (imaginary part ofthe complex refractive index) in the range of 390 to 420 nm.

In the present invention, it is preferred that the recording layer (3)further contain, aside from the trimethine cyanine dyes, a singletoxygen quencher. It is also preferred that the recording layer (3)contain a singlet oxygen quencher anion in the form of ionically-bondedcompound formed with a cation dye.

Preferred examples of the quencher include metal complexes ofacetylacetonato quenchers, bisdithiol quenchers, such asbisdithio-alpha-diketone and bisphenyldithiol quenchers, andthiocatechol quenchers, salicylaldehyde-oxime quenchers, andthiobisphenolate quenchers. Also preferred are amine compoundscontaining radical cation of nitrogen and amine quenchers such assterically hindered amines.

A preferred dye to form the ionically-bonded compound is a cyanine dyehaving an indolenine ring. A preferred quencher is a metal complex dyesuch as bisphenyldithiol metal complex.

The quencher and the cyanine dye may be added to the recording layer (3)either individually or in the form of ionically-bonded compound. Ineither case, the quencher is added preferably in an amount of 1 mole orless, and particularly in an amount of about 0.05 to about 0.8 moles,with respect to 1 mole of the total cyanine dye. In this manner, thelight resistance of the recording layer (3) is increased.

Preferably, the recording layer (3) is formed using spin-coat technique.Specifically, a coating solution, which has been prepared by dissolvingthe aforementioned cyanine dye and, if necessary, the singlet oxygenquencher in a suitable solvent, is spin-coated onto the supportingsubstrate (2) and is dried, when necessary. The screen-printingtechnique, the dipping technique, or other suitable coating techniquemay also be employed.

The organic solvent used in the coating solution for forming therecording layer (3) may be suitably selected depending on the type ofthe dye used and may be an alcohol, ketone, ester, ether, aromaticsolvent, fluorinated alcohol, or a halogenated alkyl. A preferredexample of the organic solvent is 2,2,3,3-tetrafluoropropanol.

The recording layer (3) is 30 to 120 nm thick, and preferably 40 to 80nm thick, in the land area. A proper thickness of the recording layermay be determined by taking into account factors including the desiredreflective index, degree of modulation, and the heat interference withthe adjacent tracks and marks. Among parameters known to affect thesefactors are geometry of the substrate, behavior of the thermallydegrading dye, optical characteristics of the dye, opticalcharacteristics and heat conductivity of the adjacent layers.

Preferably, a dielectric layer (4) is deposited over the recording layer(3). The dielectric layer (4) not only serves to provide mechanical andchemical protection for the recording layer (3) but also serves as aninterference layer for adjusting optical characteristics of therecording layer (3). The dielectric layer (4) may consist of a singlelayer or a plurality of layers.

Formed on top of the recording layer (3), the dielectric layer (4) musttransmit the recording/reproducing laser light of 390 to 420 nm.Preferably, the dielectric layer (4) has a refractive index n₄ (realpart of the complex refractive index) of 2 or higher with respect to thewavelength of the recording/reproducing laser light. The refractiveindex n₄ that is higher than 2 is preferred since it allows easyadjustment of the reflective index of the optical recording medium to adesired range (for example, 15 to 20%). While there is no specific upperlimit set for the value of n₄, materials known to transmit light of 390to 420 nm generally have a refractive index of approximately 3. It isalso preferred that the dielectric layer has an extinction coefficientk₄ (imaginary part of the complex refractive index) of 0.2 or lower withrespect to the recording/reproducing laser light. The extinctioncoefficient k₄ that is 0.2 or lower is preferred since it leads to areduced energy absorption by the dielectric layer, allows a wider marginfor the adjustment of the reflective index of the medium, and providesan increased sensitivity. Though not limited to a particular value, thelower limit of the extinction coefficient k₄ is approximately 0.

The dielectric layer (4) may be made from an oxide, nitride, sulfide,fluoride, or a composite material thereof of at least one metal selectedfrom the group consisting of Si, Zn, Al, Ta, Ti, Co, Zr, Pb, Ag, Zn, Sn,Ca, Ce, V, Cu, Fe, and Mg. It is particularly preferred that thedielectric layer (4) is made from ZnS—SiO₂, AlN, or Ta₂O₃ in view of theabove-described preferred values for the refractive index n₄ and theextinction coefficient k₄. As for ZnS—SiO₂, the SiO₂ content ispreferably in the range of 10 mol % to 40 mol %. The dielectric layercan be formed using techniques such as the ion-beam sputtering, thereactive sputtering, and the RF sputtering techniques. A propertechnique that does not cause damage to the recording layer is selectedfrom these techniques.

While not limited to a particular value, the thickness of the dielectriclayer (4) is for example from about 20 to about 150 nm, preferably from30 to 70 nm. If less than 20 nm thick, the dielectric layer (4) mayallow penetration of some components of the light-transmitting layer (5)to the recording layer (3). In contrast, the dielectric layer (4), ifmore than 150 nm thick, will exhibit too high a heat conductivity, whichmay result in a reduced sensitivity.

A light-transmitting layer (5) is formed on the dielectric layer (4), orin the absence of the dielectric layer (4), on the recording layer (3).

The material for the light-transmitting layer (5) may be selected fromUV-curable resins, electron beam-curable resins, thermosetting resins,or other proper resins, as long as these materials are opticallytransparent, show low absorbance and low reflectivity for the wavelengthrange of the laser light used (i.e., 390 to 420 nm), and have a lowbirefringence. Those materials that are curable by an activation energyray, including UV-curable resins and electron beam-curable resins, areparticularly preferred. These materials are preferably of non-solventtype.

Specifically, the activation energy ray-curable material is made from aUV (or electron beam)-curable compound or a polymerization compositionthereof. Examples of such materials include monomers, oligomers, andpolymers that, within their molecules, contain or incorporate bonds thatform crosslinks, or cause polymerization, upon exposure to UV-light,including acrylic double bonds found in compounds such as estercompounds of acrylic acid and methacrylic acid, epoxy acrylate, andurethane acrylate; allylic double bonds found in compounds such asdiallylphthalate; and unsaturated double bonds found in compounds suchas maleic acid derivatives. These compounds are preferablypolyfunctinal, and particularly trifunctional or higher, and may be usedindividually or in combination of two or more compounds. The compoundsmay include those that are monofunctional.

Preferably, the UV-curable monomers are compounds with a molecularweight of less than 2000, and the UV-curable oligomers are compoundswith a molecular weight of 2000 to 10000. Examples of these compoundsinclude styrene, ethyl acrylate, ethyleneglycol diacrylate,ethyleneglycol dimethacrylate, diethyleneglycol diacrylate,diethyleneglycol dimethacrylate, 1,6-hexaneglycol diacrylate, and1,6-hexaneglycol dimethacrylate. Among particularly preferred arepentaerythrytol tetra(meth)acrylate, pentaerythrytol (meth)acrylate,trimethylolpropane tri(meth)acrylate, trimethylolpropanedi(meth)acrylate, and (meth)acrylates of phenolethyleneoxide adducts.Other UV-curable oligomers include modified acrylic compounds ofoligoester acrylates and urethane elastomers.

The UV (or electron beam)-curable material may contain a knownphotopolymerization initiator. The photopolymerization initiator isnecessary when UV-ray is used as the activation energy ray but not whenan electron beam is used. The photopolymerizatoin initiator may besuitably selected from those commonly in use, including acetophenones,benzoins, benzophenones, and thioxanthones. Some of thephotopolymerizaton initiators are known to act as photoradicalinitiators. Examples include Darocure 1173, Irgacure 651, Irgacure 184,and Irgacure 907 (each manufactured by Ciba Specialty ChemicalsCorporation). The photopolymerization initiator is added in an amountof, for example, 0.5 to 5% by weight with respect to the amount of theUV (or electron beam)-curable component.

Compositions containing an epoxy resin and a photo cation polymerizationcatalyst may also suitably used as the UV-curable material. Such anepoxy resin is preferably an alicyclic epoxy resin, and more preferably,an alicyclic epoxy resin having two or more epoxy groups within itsmolecule. Preferably, the alicyclic epoxy resin is at least one selectedfrom the group consisting of3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate,bis-(3,4-epoxycyclohexylmethyl)adipate,bis-(3,4-epoxycyclohexyl)adipate,2-(3,4-epoxycyclohexyl-5,5-spiro-3,4-epoxy)cyclohexane-meth-dioxane,bis(2,3epoxycyclopentyl)ether, and vinylcyclohexene dioxide. While epoxyequivalent of the alicyclic epoxy resin is not limited to a particularvalue, it is preferably in the range of 60 to 300, and more preferablyin the range of 100 to 200 since the alicyclic epoxy resin having epoxyequivalent in this range can ensure a high curability.

The photo cation polymerization catalyst may be any known catalyst usedfor this purpose. Examples include one or more of complexes of metalfluoroborates and boron trifluoride, bis(perfluoroalkylsulfonyl)methanemetal salts, aryl diazonium compounds, aromatic onium salts of elementsof group 6A of the periodic table, aromatic onium salts of elements ofgroup 5A of the periodic table, dicarbonyl chelates of elements of group3A to group 5A of the periodic table, thiopyrilium salts, elements ofgroup 6A with MF6 anion (where M is P, As or Sb), complex salts oftriarylsulfonium, complex salts of aromatic iodonium, and complex saltsof aromatic sulfonium. Preferably, one or more of complex salts ofpolyarylsulfonium, aromatic sulfonium salts or iodonium salts ofhalogen-containing complex ions, and aromatic onium salts of elements ofgroups 3A, 5A and 6A. The photo cation polymerization catalyst is addedin an amount of, for example, 0.5 to 5% by weight with respect to theamount of the UV-curable component.

Preferably, the activation energy ray-curable material used in thelight-transmitting layer (5) has a viscosity (25° C.) of 1,000 to 10,000cp.

The activation energy ray-curable material is spin-coated on thedielectric layer (4) and is subsequently exposed to UV-light or otheractivation energy light for curing to form the light-transmitting layer(5).

A resin sheet with a desired thickness to serve as thelight-transmitting layer (5) may be adhered using an adhesive, providedthat the resin and the adhesive are both optically transparent, show lowabsorbance and low reflectivity for the wavelength range of the laserlight used (i.e., 390 to 420 nm), and have a low birefringence.

Examples of the resin for use in the resin sheet include polycarbonate,amorphous polyolefin, and polyester. Prior to adherence, the resin sheetmay be subjected to annealing (thermal relaxation) in the temperaturerange of −20° C. to +80° C. with respect to the heat deformationtemperature of the resin. This is to remove the residual stressgenerated during the production of the sheet. Without annealing, theresidual stress of the sheet may cause deformation of the disk duringstorage. The annealing may be carried out by using various heatingmeans, including a heater, hot plate, hot roller, bake furnace, andelectromagnetic induction, which is properly selected for use dependingon required conditions for the process.

The adhesive for adhering the sheet may be selected frompressure-sensitive adhesives and UV-curable materials. For example, theactivation energy ray-curable material described above as a material forthe light-transmitting layer (5) can also serve as a suitable materialfor the adhesive for adhering the sheet.

When used as the adhesive, the activation energy ray-curable material isspin-coated onto the dielectric layer (4) to deposit an uncured resinmaterial layer. Before the resin cures, the sheet is placed on the resinmaterial layer and is then irradiated with UV-light or other activationenergy rays to cure the resin material layer. In this manner, theadhered sheet can serve as the light-transmitting layer (5). Morespecifically, the sheet is placed on the still uncured resin materiallayer under vacuum (0.1 atm or less), which is then raised toatmospheric pressure. UV-light is then irradiated onto the sheet to curethe resin material layer.

In general, a correlation exists among the disk skew margin θ (referredto simply as ‘skew margin,’ hereinafter), the wavelength λ of therecording/reproducing laser light, and the numerical aperture NA of theobjective lens: Japanese Patent Laid-Open Publication No. Hei 3-225650discloses the following relationship that holds between these factorsand the skew margin:θ∝λ/[t×(NA)³].

In actual mass production of optical disks, if the permissible skew isdetermined to be 0.4° in view of product yield and production cost, andgiven that short wavelength laser (λ=380 nm) is used with an objectivelens with a high numerical aperture (NA≧0.76), the light-transmittinglayer with a thickness of 170 μm or less is thin enough to ensure a skewmargin comparable to that required in DVDs.

On the other hand, the minimum thickness of the light-transmitting layer(5) is preferably 1 μm or larger in order to ensure protection of thedielectric layer (4) and the recording layer (3). However, when theresin sheet is used to serve as the light-transmitting layer (5), theminimum thickness of the light-transmitting layer (5) is preferably keptat 50 μm or larger including the thickness of the adhesive since theresin makes it difficult to form a sheet with uniform thickness. Thus,the preferred range for the thickness t of the light-transmitting layer(5) is from 1 to 150 μm when the layer is formed by coating, and from 50to 150 μm when the layer is formed by adhering a resin sheet.

EXAMPLES

The present invention will now be described in detail with reference toseveral examples, which are intended to be only illustrative and notexhaustive.

Example 1

A grooved polycarbonate substrate, 1.1 mm thick and 120 mm in diameter,was used to serve as the supporting substrate (2). Referring to theFIGURE, the groove depth (Gd) was 85 nm, the groove width (Gw) was 160nm, and the groove pitch (Gp) was 320 nm (=track pitch).

0.08 g cyanine dye, denoted as DD-1, dissolved in 9.92 g2,2,3,3,-tetrafluoropropanol was spin-coated onto the surface of thesupporting substrate (2) to form a layer serving as the recording layer(3) with a thickness of approximately 60 nm in the land area.

Analysis of the absorbance spectrum of the recording layer (3) revealedthat the recording layer (3) containing the cyanine dye DD-1 had theminimum refractive index (real part of the complex refractive index)n_(min) of 0.88 at 404 nm and a refractive index n of 0.88 and anextinction coefficient (imaginary part of the complex refractive index)of 0.46 at 405 nm.

Using RF sputtering technique, a layer of ZnS(80 mol %)—SiO₂(20 mol %)to serve as the dielectric layer (4) was formed on the recording layer(3) to a thickness or about 50 nm. The dielectric layer (4) had arefractive index (real part of the complex refractive index) n₄ of 2.3and an extinction coefficient (imaginary part of the complex refractiveindex) k₄ of 0.

A UV-curable resin (viscosity at 25° C.=5000 cp) was then spin-coatedonto the dielectric layer (4) and was subsequently irradiated withUV-light to form an about 100 μm thick layer to serve as thelight-transmitting layer (5). In this manner, a sample optical disk witha layer construction of the FIGURE was obtained.

Examples 2 to 4

In each of Examples 2 to 4, a sample optical disk was prepared in thesame manner as in Example 1, except that in place of the cyanine dyeDD-1, a cyanine dye DD-2 (Example 2), a cyanine dye AA-1 (Example 3), ora cyanine dye AA-2 (Example 4) was used.

Comparative Examples 1 to 3

In each of Comparative Examples 1 to 3, a sample optical disk wasprepared in the same manner as in Example 1, except that in place of thecyanine dye DD-1, a cyanine dye a (Comparative Example 1), a cyanine dyeb (Comparative Example 2), or a cyanine dye c (Comparative Example 3),each of which is shown below, was used.

Characteristics of the cyanine dyes used in Examples 1 to 4 andComparative Examples 1 to 3 are shown in Table 1 below.

TABLE 1 At 405 nm Characteristics in minimum value n_(min) TrimethineRefractive Extinction Wavelength λ_(min) of Refractive Extinctioncyanine index n coefficient k minimum value n_(min) (nm) index n atλ_(min) coefficient k at λ_(min) Example 1 DD-1 0.88 0.46 404 0.88 0.43Example 2 DD-2 1.20 0.93 399 1.18 0.78 Example 3 AA-1 0.95 0.51 397 0.910.36 Example 4 AA-2 1.07 0.15 417 0.98 0.41 Comparative a 1.31 0.25 4300.94 0.72 Example 1 Comparative b 1.23 0.01 453 0.78 0.38 Example 2Comparative c 1.40 0.05 453 1.04 0.37 Example 3[Recording/Reproducing Tests]

The sample optical disk prepared in Example 1 was tested for therecording/reproducing performance in the following manner:

The sample optical disk of Example 1 was mounted on an optical disktester (Product name: DDU-1000, manufactured by Pulstech Industrial Co.,Ltd.). Using an objective lens with a NA of 0.85, a recording laser beamhaving a wavelength in the blue range (405 nm) was focused by a focusinglens placed within a recording head onto the land area of the opticaldisk to effect recording/reproducing information. The laser beam wasshone from the light-transmitting layer side of the optical disk. 1.7RLL-modulated signal (8T) was used as the recording signal withinformation recorded only on one track. Multiple pulse train was usedfor recording: Setting was made in such a manner that, assuming thelength of the top pulse of the pulse train to be 1T, the length of thelast pulse was 1T and the length of each of multiple pulses between thetop and the last pulses was 0.4T (T=clock period). Information wasrecorded in such a manner that, with a recording power of 10 mW and aminimum pit length of 0.16 μm, the recording line density was 0.12 μm ofthe channel bit length/bit. The recorded information was subsequentlyreproduced with a reproducing power of 0.4 mW and, as a result, goodsignal characteristics were obtained.

Likewise, the sample optical disks of Examples 2 to 4 and ComparativeExamples 1 to 3 were each tested for the recording/reproducingperformance. Good signal characteristics were obtained in each of thesample optical disks of Examples 2 to 4, whereas the degree ofmodulation was small and the C/N ratio was insufficient after recordingin the sample optical disk of Comparative Example 1. The sample opticaldisks of Comparative Examples 2 and 3 required the recording power aslarge as about 15 mW and the sensitivity of these disks wassignificantly low. Moreover, the degree of modulation was small and theC/N ratio was insufficient after recording in these sample opticaldisks.

The above-described examples are only illustrative and are not intendedto limit the scope of the invention in any way. Further, anymodification in a scope equivalent to the claims is within the scope ofthe present invention.

1. An optical recording medium capable of being recorded and/orreproduced thereon with light of 390 to 420 nm incident upon a lighttransmitting layer surface, comprising at least: a supporting substrate;a recording layer on the supporting substrate, the recording layercontaining an organic compound as a major component; a dielectric layeron the recording layer; and a light-transmitting layer on and directlyin contact with the dielectric layer, the light-transmitting layerhaving a thickness of 1 to 150 μm and being capable of transmittinglaser light with a wavelength of 390 to 420 nm for recording andreproducing information, wherein the organic compound in the recordinglayer includes a trimethine cyanine dye that has the minimum valuen_(min) of its refractive index n (real part of the complex refractiveindex) within the range of 370 to 425 nm and has a refractive index n of1.2 or lower with respect to the wavelength of the recording/reproducinglaser light, and the organic compound, when absorbing the laser light,melts or degrades to bring about a change in the refractive index,thereby effecting recording of the information and wherein thetrimethine cyanine dye contains a trimethine chain with twonitrogen-containing heterocyclic rings positioned on ends of thetrimethine chain, one of the two nitrogen-containing heterocyclic ringsbeing benzoxazole and the other of the two heterocyclic rings beingselected from the group consisting of benzoxazole, benzimidazole andindolenine, wherein lands and grooves are formed on the supportingsubstrate with the grooves being 60 to 150 nm in depth, and having apitch of 290 to 350 nm wherein only the land area serves as a recordingarea.
 2. The optical recording medium according to claim 1, wherein, atthe wavelength of the reproducing laser light, the melting or thedegradation of the organic compound causes an increase in the refractiveindex n of the organic compound.
 3. The optical recording mediumaccording to claim 1, wherein the organic compound has an extinctioncoefficient k (imaginary part of the complex refractive index) of 0.15or above, with respect to both the wavelength of the recording laserlight and the wavelength of the reproducing laser light.
 4. The opticalrecording medium according to claim 1, wherein the other of the twoheterocyclic rings being benzoxazole.
 5. The optical recording mediumaccording to claim 1, wherein the recording layer contains, in additionto the organic compound, a quencher.
 6. The optical recording mediumaccording to claim 1, wherein the organic compound in the recordinglayer has an extinction coefficient k (imaginary part of the complexrefractive index) of 0.15 or above with respect to the wavelength ofboth the recording and reproducing laser light.
 7. The optical recordingmedium according to claim 6, wherein the extinction coefficient k(imaginary part of the complex refractive index) is in the range of 0.3to 0.95 with respect to the wavelength of both the recording andreproducing laser light.
 8. The optical recording medium according toclaim 7, wherein the extinction coefficient k (imaginary part of thecomplex refractive index) is in the range of 0.4 to 0.8 with respect tothe wavelength of both the recording and reproducing laser light.
 9. Theoptical recording medium according to claim 1, wherein the dielectriclayer has refractive index n₄ (real part of the complex refractiveindex) of 2 or higher and an extinction coefficient k₄ (imaginary partof the complex refractive index) of 0.2 or lower with respect to thewavelength of the recording/reproducing laser light.
 10. The opticalrecording medium according to claim 1, wherein the refractive index n is1.1 or lower with respect to the wavelength of the recording/reproducinglaser light.
 11. The optical recording medium according to claim 10,wherein the refractive index n is 1.0 or lower with respect to thewavelength of the recording/reproducing laser light.
 12. The opticalrecording medium according to claim 1, wherein the trimethine cyaninedye has the following general formula (I):

wherein Y represents a hydrogen atom, a halogen atom, a lower alkylgroup, or a phenyl group; X represents an anion, which is a halogen ion,ClO₄ ⁻, BF₄ ⁻, PF₆ ⁻, SbF₆ ⁻, or SCN⁻; m is 0 or 1; the Q-containingring has the following formula (A), and the Q′-containing ring has oneof the following formulae (A), (B) or (C):

wherein R₁ is a substituted or unsubstituted alkyl group having 1 to 4carbon atoms; R₂, R₃, R₄ and R₅ may or may not be identical to oneanother and each independently represent a hydrogen atom, alkyl group,nitro group, alkoxy group, or a halogen atom; R₆ represents a methyl orethyl group; R₇ and R₈ may or may not be identical to one another andeach independently represent a methyl or ethyl group.
 13. The opticalrecording medium according to claim 12, wherein the trimethine cyaninedye is selected from the group consisting of


14. The optical recording medium according to claim 1, wherein therecording layer and the supporting substrate are adjacent to each other.15. An optical recording/reproducing method, comprising the steps of:providing an optical recording medium comprising at least a supportingsubstrate; a recording layer on the supporting substrate, the recordinglayer containing an organic compound as a major component; a dielectriclayer on the recording layer; and a light-transmitting layer on anddirectly in contact with the dielectric layer, the light-transmittinglayer having a thickness of 1 to 150 μm and being capable oftransmitting laser light with a wavelength of 390 to 420 nm forrecording and reproducing information, wherein the organic compound inthe recording layer includes a trimethine cyanine dye that has theminimum value n_(min) of its refractive index n (real part of thecomplex refractive index) within the range of 370 to 425 nm and has arefractive index n of 1.2 or lower with respect to the wavelength of therecording/reproducing laser light, and the organic compound, whenabsorbing the laser light, melts or degrades to bring about a change inthe refractive index and wherein the trimethine cyanine dye contains atrimethine chain with two nitrogen-containing heterocyclic ringspositioned on ends of the trimethine chain, one of the twonitrogen-containing heterocyclic rings being benzoxazole, and the otherof the two heterocyclic rings being selected from the group consistingof benzoxazole, benzimidazole and indolenine; irradiating a recordinglaser light of 390 to 420 nm onto the optical recording medium from thelight-transmitting layer side thereof to effect recording of theinformation, whereupon the refractive index n of the organic compoundwith respect to the wavelength of reproducing laser light of 390 to 420nm is raised in the area irradiated with the recording laser light; andsubsequent to the recording step, irradiating the reproducing laserlight of 390 to 420 nm onto the optical recording medium from thelight-transmitting layer side thereof to effect reproducing of theinformation wherein lands and grooves are formed on the supportingsubstrate with the grooves being 60 to 150 nm in depth, and having apitch of 290 to 350 nm wherein only the land area serves as a recordingarea.